CHICAGO (AP) - Responding to deepening mistrust of one of the nation's largest police forces, the federal government opened an investigation Monday into the Chicago Police Department, and authorities announced they would not charge an officer in the death of a 25-year-old black man who was shot in the back last year.

The Justice Department investigation was to look into patterns of racial disparity in the use of force. It comes nearly two weeks after the release of a video showing a white Chicago police officer shooting a black teenager 16 times.

Lack of trust between police and their communities, "makes it more difficult to gain help within investigations, to encourage the victims and the witnesses of crime to speak up and to fulfill the most basic responsibilities of public-safety officials," Attorney General Loretta Lynch said. "And when suspicion and hostility is allowed to fester, it can erupt into unrest."

The investigation, which is separate from an existing federal investigation into last year's shooting death of 17-year-old Laquan McDonald, will also review how the department disciplines officers and handles misconduct accusations. Justice Department officials say they use so-called patterns-and-practices probes to identify systemic failings in troubled police departments and to improve trust between police and the communities they serve.

The civil-rights investigation follows recent ones in Baltimore and Ferguson, Missouri, and comes as the police department and Mayor Rahm Emanuel are under intense scrutiny over their handling of the October 2014 death of McDonald. Officer Jason Van Dyke was charged with first-degree murder Nov. 24, more than a year after the killing and just hours before the release of police dashboard camera footage showing the officer shooting the teenager.

Emanuel, who initially said a federal civil rights investigation would be "misguided" but later reversed course, said the city needs comprehensive solutions in the wake of the video showing McDonald's death.

Speaking at a news conference, Emanuel said the police department's challenges go beyond one case and he's making several reforms, including appointing a new leader for the Independent Police Review Authority, which investigates shootings by police. The previous head resigned Sunday.

The authority's new chief will be Sharon Fairley, a former federal prosecutor who also worked with Chicago's Office of Inspector General. She appeared with Emanuel at City Hall and said she has no agenda beyond the pursuit of integrity and transparency.

Also Monday, Cook County State's Attorney Anita Alvarez said there would be no charges against Officer George Hernandez in the shooting of Ronald Johnson. Authorities say he pointed a gun at police before he was killed on Oct. 12, 2014.

Alvarez and Assistant State's Attorney Lynn McCarthy spent more than 30 minutes detailing evidence before showing the dashcam video, which similarly to the McDonald case has no audio. The state's attorney's office overlaid police radio communications.

The video shows Johnson running from police across a street with several officers in pursuit, and then one officer fires. Johnson is not on screen when he was struck by two bullets.

The video was also slowed down to show what McCarthy said was a gun in Johnson's hand. Prosecutors say a loaded weapon was found in his hand after he was killed. She also said Johnson ignored officers' commands to stop and drop his weapon and had been in a physical altercation with at least one other officer before he was shot.

"We're in different times right now when we're talking about transparency and what the public wants to see," Alvarez said after showing the video. "I have pretty much opened the door here."

The attorney for the Johnson family, Michael Oppenheimer, said the prosecutors' investigation was a "joke" and an affront to Johnson's family and Cook County citizens.

When asked why there was no audio on this video and others, Alvarez said: "That's a problem for the Chicago Police Department, and I think they need to answer to that. ... Time and time again we look at these videos, and there is not any audio."

Alvarez has been criticized for not filing charges earlier in the McDonald case, in which the video shows the teen veering away from officers on a four-lane street when Van Dyke, seconds after exiting his squad car, opens fire from close range. The officer continues shooting after McDonald crumples to the ground and is barely moving.

The Chicago City Council signed off on a $5 million settlement with McDonald's family even before the family filed a lawsuit, and city officials fought in court for months to keep the video from being released publicly. The city's early efforts to suppress the footage coincided with Emanuel's re-election campaign, when the mayor was seeking African-American votes in a tight race.

Since the release of the McDonald video, Emanuel forced Police Superintendent Garry McCarthy to resign and formed a task force to examine the police department. The department's chief of detectives also resigned Monday.

But despite the changes Emanuel has implemented, protesters' calls for him to resign - something he said he won't do - have grown louder.

Politicians, including Democratic presidential candidate Hillary Clinton and Illinois Attorney General Lisa Madigan, called for the federal civil rights investigation. The Rev. Jesse Jackson said he hopes it would focus not only on the police department, but on Emanuel's office and the Cook County State's Attorney's office.

"All three of them - the police, City Hall and the prosecutor's office - are suspect," Jackson said. "We cannot trust them."

The Justice Department has opened 23 investigations of police departments since the start of the Obama administration.

If the Justice Department finds systemic violations, the investigations typically result in court-enforceable agreements between the federal government and the community that serve as blueprints for change and are overseen by an independent monitor. The federal government has the option of suing a police department that is unwilling to make changes.